The relative NOx performance of Pd-only and Pd-Rh catalysts has been investigated under a series of operating conditions on a vehicle and in a laboratory reactor. The vehicle data indicates that the choice of Pd-only vs. Pd-Rh technology should be specific to the operating conditions found on that vehicle. Under low temperature conditions, Pd-Rh and Pd-only catalysts have similar NOx performance attributes. However, high temperature portions of the drive cycle accentuate the differences between Pd-only and Pd-Rh catalysts and lead to a large NOx performance advantage for the Pd-Rh catalyst. Laboratory reactor data indicates that these differences in activity are tied to the poisoning effects of reduced sulfur species on Pd, which become more severe as the temperature is increased and as the gas-phase stoichiometry becomes richer.